We use a variant of the Duration Calculus to design a chemical
concentration control system which has both nontrivial dynamics and
control programs.  The system is developed by refinement along the
lines proposed in formal methods of software construction.  Refinement
rules for durational specification formulae are investigated for this
purpose. The overall specification is refined into several lower level
specifications. One such lower level specification is implemented by
software and a control program is proposed and shown to be correct.